This invention relates in general to metrology systems for measuring periodic structures such as overlay targets employed in photolithography in a research or production environment, and, in particular, to a metrology system employing optical phase for detecting misalignment of such structures.
Overlay error measurement requires specially designed marks to be strategically placed at various locations, normally in the street area between dies, on the wafers for each process. The alignment of the two overlay targets from two consecutive processes is measured for a number of locations on the wafer and the overlay error map across the wafer is analyzed to provide feedback for the alignment control of lithography steppers.
A key process control parameter in the manufacturing of integrated circuits is the measurement of overlay target alignment between successive layers on a semiconductor wafer. If the two overlay targets are misaligned relative to each other, the electronic devices fabricated will malfunction and the semiconductor wafer will need to be reworked or discarded.
Typically, conventional overlay targets are box-in-box targets and bar-in-bar targets. The box-in-box target typically has a 10 xcexcm inner box and a 20 xcexcm outer box. The outer box is printed on the substrate (or previous process layer) and the inner box is resist printed on the current layer. Overlay error is reported as the mis-position of the inner mark with respect to the outer mark. A bar-in-bar target also has a 10 xcexcm inner target on the current layers and a 20 xcexcm outer target on the previous layers. However, the box edge is replaced with a narrow bar 2 xcexcm wide. The box-in-box targets are more compact; however, the bar-in-bar targets provide better measurement performance. Overlay targets may comprise grating structures on top of the wafer or etched into the surface of the wafer. For example, one overlay target may be formed by etching into the wafer while another adjacent overlay target may be a photoresist layer at a higher elevation over the wafer.
Conventional systems for detecting overlay target misalignment typically employs an electronic camera that images the xe2x80x9cbox-in-box target. xe2x80x9d The accuracy of the conventional system is limited by the accuracy of the line profiles in the target, by aberrations in the illumination and imaging optics and by the image sampling in the camera. Such methods are complex and they require full imaging optics. Vibration isolation is also required and it may be difficult to integrate such systems into process equipment.
An improvement to the conventional method is described in U.S. Pat. No. 6,023,338. This patent discloses a method where two overlay target structures are placed next to each other and two radiation beams are scanned in two separate paths across portions of both structures. The intensity of the radiation reflected along both paths are detected and processed to calculate any offset between the two structures.
While the above-described improved method may be useful for some applications, it requires beams to be scanned across periodic structures such as overlay targets. It is desirable to develop an improved system with better performance and simplified scanning characteristics.
This invention is based on the observation that by utilizing optical phase detection, high sensitivity for detecting misalignment of periodic structures can be achieved. Thus, two periodic structures such as overlay targets are placed side-by-side so that they are periodic substantially along the same direction, where portions of both structures are illuminated by coherent radiation. The size(s) of the beam(s) illuminating portions of the structures are large enough to generate diffraction signals by the structures. These diffraction signals are caused to interfere leading to the detection of optical phase which is a measurement of the misalignment between the structures. The misalignment may then be used to control lithographic instruments such as a lithographic stepper or to determine whether or not the patterns of the structures are correctly placed and will yield functional devices.
When the paths of radiation traveling between the radiation source, the structures and detectors are close together, the phase sensitive detection is less sensitive to environmental factors such as vibration and thermal drifts. Since the system employs larger spot illumination, the optics of the system are less sensitive to focus accuracy. The system is compact and readily integratable with process equipment. Due to the enhanced sensitivity compared to conventional systems, the system is able to detect misalignment of periodic structures that are low contrast.